Nowadays, popular applications on smartphones have much more sophisticated traffic pattern than what the network architect originally had in mind. It turns out that the existing networks do not support popular applications that well. For example, chattiness of applications where traffic is based on user interaction results in inconstant QoS requirement over time. Another example is “keep alive messages” or background traffic of application or OS where traffic has short and infrequent data sessions. When bringing this type of traffic, it creates a number of issues. First, many always-on applications generate frequent traffic, e.g., for keep alive and status update, which brings significant problems. Second, the signaling in the network has increased a lot due to frequent context establishment and release, e.g., for Connected—Idle transitions. In many networks, this becomes a severe dimensioning problem for the whole network. Third, the UE batter life is short. Finally, for devices that generate always-on sparse traffic, the overhead is very large as compared to the data payload transmission.
In 3GPP LTE/LTE-A systems, operations could be divided to two radio resource control (RRC) states: RRC_CONNECTED and RRC_IDLE. In RRC_CONNECTED mode, an eNB would keep UE's context (security, id) and process radio resource management (RRM) for that UE. RRM here includes data scheduling, link monitoring (MCS adaption), handover, etc. A UE is ensured to make seamless data transmission with eNB when the UE is in RRC_CONNECTED mode. The eNB may command UE to perform RRM measurement and make handover (HO) decisions after receiving reports that indicate serving cell's signal quality is not good. Inter-eNB negotiation on exchanging UE's information would be handled in RRC_CONNECTED mode.
Since radio resources are limited and network loading is also restricted (by buffer size, backhaul capacity), it is impossible to keep all UEs in RRC_CONNECTED mode. An eNB may release part of UEs and command them to go to RRC_IDLE mode. The release decision may depend on (1) how long a UE does not have any activity, (2) UE's access priority, and (3) QoS. Once a UE goes to RRC_IDLE mode, eNB would release UE information. A UE would perform cell reselection when it finds its camped cell is getting weaker and try to re-synchronize with a new cell. An idle UE will try to re-establish its RRC connection when data arrives. The procedures comprise: a) random access process to synchronize the uplink timing, b) capabilities negotiations authorizations, and network perform admission control, and c) set up operating parameters if UE is allowable to access the network. For uplink transmission, the UE would further send buffer status report (BSR) to request UL grant.
Based on current LTE specification, it is expected that an RRC transition would consume a lot of signaling and cause a lot of delay. When traffic is spare and small, it would be very inefficient to make the transmission since most UE would be kept in IDLE and need to re-establish the RRC connection. As a result, LTE/LTE-A system needs an enhancement to deal with the problem.
It has been shown that although there has only 30% data traffic oriented from smartphone device, they would contribute 80% signaling traffic. This means that new data services have significant impacts on User-plane and Control-plane performance. Unfortunately, the impacts could not be handled well by conventional protocol, and system capacity would be reduced. Furthermore, it has been observed that background traffic is usually small and arrives at long interval while Discontinuous Reception Mechanism (DRX) may not work efficiently under this profile and increase frequent RRC state transition. As a result, UE battery life would be degraded while running those smartphone applications.
To solve the problems, two issues shall be studied. First, how to decide when to send UE to RRC_IDLE mode. In general, the decision relies on a proper RRC release timer that is controlled by eNB. The timer is re-started upon transmitting each packet, and if the timer is expired, UE is released. The timer ideally shall be configurable based on running applications, but how to achieve the objective is unclear. Second, how to decide when to resume RRC connection. Traditionally, when a packet arrives at UE buffer, UE will decide to access with eNB to transmit that packet as soon as possible. However, considering delay-tolerant data applications, it is not necessary to trigger the transmission right away. Therefore, it is proposed that UE can have intelligence and mechanism to decide whether to postpone the transmission and when the transmission shall happen for additional power saving.